2. File Systems
File organization
Access methods
Directory Organization: single, two-level,
hierarchy
File system and directory implementation
Allocation schemes : contiguous, linked,
indexed
3. File system
In a computer, a file system (sometimes written
filesystem) is the way in which files are named and
where they are placed logically for storage and retrieval.
The logical unit within a file system is the file
logical files are mapped into physical entities by the OS
in user's view, file is the smallest unit that can be saved
to disk
4. A file system defines the structure and the rules used to
read, write, and maintain information stored on a disk.
Which system used is determined by;
Hardware
Software
Security needs
Need for a dual-booting system
5. File Attributes
name : provides handle for reference
• DOS (8 chars + 3 char extension), Windows
(unlimited? length)
• UNIX (spaces tricky, no extension needed)
type : indicates how the file should be treated
• DOS/Windows rely on extension, can map extensions
to programs
• Mac associates creator attribute with each file (OS X?)
• UNIX uses "magic number", first few bytes of file
specify file type
6. protection : permissions, access control information
• UNIX utilizes permission string: chmod 644 foo.txt
-rw-r--r—
owner & group: chown, chgrp
• Windows utilizes file properties/attributes: NoAccess,
List, Read, Read & Add, …
Location & Size
Accounting Information
7. e.g. MS-DOS filename
Filename: 1 to 8 characters
File extension: 1 to 3 characters
DOS only uses uppercase
You cannot use spaces
A single period separates the file extension from the
filename
You cannot use these characters:
/ : * ? “ < > | + = ; , [ ]
8. The Full Path
What does it mean?
The complete path from the root of the files system
to the desired object.
Use (vs. /)
C: - the root
For Example;
E:BSIT6th SemesterSystem AdministrationFile
System.ppt
9. File Names continued
Filenames consist of two parts
Main part of filename
Identifies the contents of the file
File extension
Identifies the file type
The last period separates the file extension from the
main part of the filename.
Windows typically associates a file extension with an
application installed on a computer
10. Folder Names
Folder names follow the same guidelines for naming files
Folder names usually do not have a file extension
11. File Operations
create : find space on disk and make entry in directory
write : write to file, requires positioning within the file
read : read from file, involves positioning within the file
delete : delete directory entry, reclaim disk space
reposition : move read/write position
12. the OS must maintain
information about all open files
file pointer : the current position of the read/write
pointer in the file
disk location : the location of the file on the disk
file open count : keep track of number of processes
currently accessing the file
such a table of information allows the OS to enforce
policies such as only one process can write to a file at a
given time
13. File Structure
files can be stored physically as
Bytes
Lines
records
whatever entity is stored, OS must map into
a disk sector
because on a physical disk, sectors are
smallest writeable unit
14. access methods:
sequential : information in the file is accessed from first
to last
readNext , writeNext , reset
direct : possible to reposition read/write pointer to any
position
such files are generally made up of fixed-length records
readRecord N, writeRecord N, positionAt N, reset
indexed : built on top of direct access, but accesses
records in file using a key
each record has a key associated with it, an index of
keys is stored with the file
readRecord KEY, writeRecord KEY, positionAt KEY, reset
15. FAT???
file allocation table – where the OS records how the disk
space is used
locates the file allocation table near the beginning of the
volume
the location of the FAT is specified in the boot sector
(BIOS Parameter Block)
actually, 2 copies of the FAT are stored for redundancy
the FAT number refers to the number of bits per table
entry
16. File Systems
FAT12: The earliest version the file system, FAT12
allows a partition to contain up to 4096MB/ (212)clusters.
FAT 16: oldest, created for DOS, supported by most
OS’s, cannot be installed on partitions larger than 2 GB, or on
hard drives larger than 4GB.
FAT 32: supports disks from 512MB to 2TB, compatible
with Windows 98 and up
17. Contd..
the FAT number refers to the number of
bits per table entry
FAT12 -> 212 = 4M different clusters can
be addressed (used for floppy disks)
FAT16 -> 216 = 64M different clusters can
be addressed (MS-DOS compatible)
FAT32 -> 228 = 256G different clusters (4
bits are reserved)
18. Advantages of FAT File System
The FAT file system is best for drives and/or partitions
under approximately 200 MB
It is better to format system partition as FAT
19. Disadvantage of FAT File System
It is not better to use FAT on partitions that are greater
than 200 megabytes. FAT partitions are limited in size to a
maximum of 4 Gigabytes (GB) under Windows NT and 2
GB in MS-DOS.
Any FAT partitions that use DOS-based disk compression
(such as DriveSpace) will not have readable files when
running Windows NT
The FAT file system is also prone to fragmentation
.
20. NTFS
New technology file system
Better file security (Encrypting File
System),
Disk compression- can compress a
file/folder, any Windows app will
automatically expand as needed
Disk Quota features- Can enforce quotas
on disk usage
21. Contd..
Recovery features-each file operation
broken down into atomic transactions.
maintains a transaction log – updates disk
after each transaction
if failure occurs during a transaction, info is
sufficient to complete or rollback
if a bad sector is found when writing, will
automatically map to a different sector
22. NTFS volumes can not be accessed by
DOS,
or Windows 95 or Windows 98.
23. Advantages of NTFS
Large disks and large files: NTFS is best for
use on volumes of about 400 MB or more
Recoverability: The recoverability designed into
NTFS is such that a user should never have to run any
sort of disk repair utility on an NTFS partition.
Security: NTFS uses the Windows NT object model
to enforce security. An open file is implemented as a file
object with a security descriptor that defines its security
attributes.
24. General indexing facility: NTFS associates
a collection of attributes with each file. The set of files in
the file management system is organized as a relational
database, so that files can be indexed by any attribute.
25. Disadvantages of NTFS
It is not recommended to use NTFS on a volume that is
smaller than approximately 400 MB
Currently, there is no file encryption built into NTFS –
encryption is available as an external option .
It is not possible to format a floppy disk with the NTFS
file system
26. File Systems
² The operating system keeps track of data (documents,
pictures, etc.) by placing it into a file.
² To store and retrieve files:
² Disk divided into tracks
² Tracks are divided into sectors
² Sectors grouped into clusters
²Number of sectors in a cluster is determined by
²Size of the hard drive
²File allocation system – FAT, FAT32, NTFS
28. Clusters?
A cluster, also known as an allocation unit, consists of
one or more sectors of storage space, and represents
the minimum amount of space that an operating system
allocates when saving the contents of a file to a disk.
The number of sectors per cluster is dependent on
Type of disk (floppy disk, hard disk)
Version of operating systems
Size of disk
Every sector contains 512 bytes. (NTFS does allow you
to change this number.)
The number of clusters per disk is determined by the
filing system (FAT 16, FAT 32 or NTFS).
29. FAT 16 FAT 32 NTFS
DRIVE SIZE
Cluster Size Cluster Size Cluster Size
260 to 511 MB 8 KB Not Supported 512 bytes
512 to 1023 MB 16 KB 4 KB 1KB
1024 MB to 2 32 KB 4 KB 2 KB
GB
2 to 4 GB 64 KB 4 KB 4 KB
4 to 8 GB Not Supported 4 KB 4 KB
8 to 16 GB Not Supported 8 KB 4 KB
16 to 32 GB Not Supported 16 KB 4 KB
>32 GB (up to 2 TB) Not Supported 32 KB 4 KB
30. OS and File System Compatibility
Operating System FAT16 FAT32 NTFS
Windows XP
Windows 2000
Windows NT
Windows 95, 98, ME
Windows 95
MS-DOS
31. What happens during the
Formatting Process?
OS creates four tables in the 1st sectors
Boot Record – the name & version number of the OS,
info. on the physical characteristics of the disk
Master File Table #1- keeps track of
Available clusters
Clusters that contain data
Clusters that are defective
Clusters that contain OS files
Master File Table #2 – copy of MFT #1
Directory Table – top level folder and file information
32. Formatting Process, cont.
Full Format
lays down new tracks and sectors
Verifies the integrity of each sector
By doing a surface scan – OS will put dummy data
into sectors and then try to read the sector
Quick Format
Removes files
Does not check for defective sectors
34. Registered Files
A file that is associated with an application on your
computer via its file extension.
Where is this information kept?
THE REGISTRY – a database that consists of a set of
files where Windows XP stores your computer’s
hardware, software, network, security,user settings or
profiles, and property settings for folders and programs.
35. Compare FAT and NTFS File
System
Compatibility
Only Windows NT supports NTFS partitions , Windows
NT and Windows 95 supports both NTFS and FAT
Volume size
FAT supports partition sizes only up to 2 GB , the
maximum size of an NTFS partition is 16 EB
36. Fault tolerance
Windows NT offers software support for several
alternate disk-access methods that increase speed
and/or fault tolerance, While FAT does maintain two
copies of the file-allocation table, in case one copy is
damaged, it’s incapable of automatically fixing errors)
File compression
NTFS has its native support for file compression, It
offers you the chance to compress individual files and
directories of your choice
37. The system partition
A better solution is to format your system partition as
FAT because NTFS partitions are accessible only via
Windows NT. If you have a fatal error with Windows NT,
you can’t simply boot a system disk to a command
prompt and fix a problem on an NTFS partition
Converting to NTFS
In MS-DOS type command: CONVERT drive: /FS:NTFS
38. Security
NTFS has a built-in security system , FAT has no local
protection, it only has the share permission (protect the
file from network
39. Ext3 third extended file
system
Stephen Tweedie
November,2001
Journaled file system commonly used by linux.
(journaling capability means no worrying about
metadata corruption. What is most noticeable is that you
can switch back and forth between ext2 and ext3 on a
partition without any problem: it is just a matter of
giving the mount command the right file system type. )
40. Advantages
(over ext2 file system)
Simple
Have the facility of backup and restore data
journaling improves reliability
indexing for larger directories.
42. Journaling levels
There are three levels.
Journal (lowest risk)
Ordered (medium risk)
Write back (highest risk)
43. Both metadata and file contents are written to the journal
before being committed to the main file system.
Only metadata is journaled; file contents are not, but it's
guaranteed that file contents are written to disk before
associated metadata is marked as committed in the journal.
Only metadata is journaled; file contents are not. The
contents might be written before or after the journal is
updated.